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The aspect of this not mentioned on the Cambridge website is that once you are trained up on a course and become an assistant tutor the centre then needs to oversee your progress to main course tutor as well (which takes another three or four courses).
This is going to be a stumbling block for most centres (and is for us) in that they won't have need for a tutor for so many courses.
So basically the only way to be trained up is to work for a centre that trains up tutors.